Three studies of health behavior change among persons at high risk of cancer are proposed. The relationship between personal evaluations of the high risk situation and behaviors to avoid incidence or mortality of cancer are examined in each study. The influence of experimental interventions to alter screening and preventive behavior are evaluated. Three populations will be studied. 1) Unaffected twins of cancer cases identified through the International Twin Study will be studied to determine behavior change among persons with close familial cancer incidence. A mass mailed intervention will be tested experimentally to determine the influence on screening or prevention behavior. 2) First degree relatives of bilateral premenopausal breast cancer cases will be studied to determine their screening behavior, their receptivity to participating in a screening program, and the effect on stage of diagnosis of screened, unscreened, and out-of-area subjects. 3) Gay men who have persistent generalized lymphadenopathy (HTLV-III positive) but not diagnosed with AIDS and healthy gay men will be surveyed to identify the extent and determinants of sexual behavior change and drug use behavior change.